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node distance = 2cm and 2cm,
     N/.style = {circle, draw, font=\Large, 
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       every edge/.style = {draw, -Stealth, semithick, shorten >=2pt},
every edge quotes/.style = {auto=right, inner sep=2pt}
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% \node (m) at (0,0) {【例3.7】$\rm{A=\left\{a b^{n} c,(a b)^{n} \mid n \geqslant 0\right\}}$FA的构造:}; 
\node (n) at (0,0){方法一: \color{red}联合式};
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\node[N] (b)[right of=a] {2};
\node[N] (c)[right of=b][label=right :{-}]{3};
\node[N] (d)[right of=c][label=left :{+ -}]{4};
\node[N] (e)[right of=d]{5};
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(a) edge [above] node {a} (b)
(b) edge [above] node {c} (c)
    edge [loop above] node {b} ()
(d) edge [above] node {a} (e)
(e) edge [bend right] node [above] {b} (d)
    ;
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